Abstract

Gold mineralisation in northeast Tasmania occurs mainly within quartz veins
hosted by a thick sequence of folded Ordovician to Devonian turbidites known
as the Mathinna Group. This sequence was intruded by post orogenic
Devonian granitoids which form the Blue Tier, Scottsdale and Eddystone.
Batholiths. Gold mineralisation has a close spatial association with the
granitoids in some areas but the majority of mineralised sites are remote from
granitoid outcrop.
Potential field geophysical methods were applied to assess the spatial
relationship between gold mineralisation, regional structure and granitoids.
Regional data were used to examine and test tectonic models for the
development of the region. Semi-regional and detailed data were used to
assess the local structural setting.
ii
Analysis of the regional gravity and magnetic fields indicates that the lower
Palaeozoic rocks of eastern Tasmania were thrust to the west over western
Tasmanian sequences. The broad magnetic anomaly west of Bridport suggests
that Cambrian basic and ultrabasic rocks underlie the Mathinna Group and the
granitoids. The gold mineralisation in the Mathinna Group may in part be
sourced from these units.
Three mineralised areas within the province were examined in detail. These
were selected as representative of a variety of geological settings and several
different styles of gold mineralisation.
In the Gladstone area gold mineralisation occurs both close to exposed
granitoids and also remote from granitoid outcrop. The magnetic data indicates
a strong structural control on the distribution of mineralisation. Many of the
mineralised sites north of Gladstone township occur on anticlinal axes and the
trend of the quartz veins in this area is parallel to northwest striking faulls
which are clearly apparent in the magnetics. Gravity data indicates that the
maximum thickness of Mathinna Group rocks in the Gladstone area is
approximately 2500 metres. There is no clear spatial relationship between the
mineralisation and the underlying granitoids. The mineralisation at Gladstone
is considered to predate the main phase of granitoid intrusion. Mineralisation in the Lisle area is closely associated with cupolas of
hornblende-biotite granodiorite which intrude the Mathinna Group.
iii
Geophysical and geochemical data indicates that these intrusions are separate
and distinct from the Diddleum Pluton which forms the western portion of the
Scottsdale Batholith. Magnetic and gravity data suggests that there are three
geophysically distinct granitoids in the Lisle area. The granodioritic Lisle
Pluton has a magnetic and a non-magnetic component but a more felsic
intrusive is inferred to underlie the Denison Goldfield. The majority of the gold
produced in the lisle area was alluvial gold recovered from the Lisle valley.
Much of this gold shows evidence of remobilisation in the placer. The primary
source of the alluvial gold at Lisle remains uncertain but the favoured
alternative is that the gold was mainly derived from quartz sulphide veins
within the Lisle Pluton and the surrounding contact metamorphosed Mathinna
Group rocks.
In the Mathinna region the majority of known gold mineralisation lies in a NNW
trending corridor between the Blue Tier and Scottsdale Batholiths. Gold
bearing quartz veins in the Mathinna Group strike either parallel or
perpendicular to the to the NNW trending regional fold axes. Geophysical data
suggest that the thickness of Mathinna Group rocks underlying the gold
corridor increases from less than 1 000 metres in the Alberton area to
approximately 3000 metres in the south near Mangana. There is no consistent
spatial relationship between the mineralisation and the underlying granitoids
although the subsurface distribution of granodioritic rocks is poorly
constrained. The main gold corridor has no magnetic expression and and
northwest trending magnetic lineaments mainly reflect lithological variations in
the Mathinna Group. Northeast striking lineaments in the Mathinna region mark
faults which have a complex history including early dextral and later sinistral
movements. The interaction of these faults with pre-existing ENE striking
fractures may have been important in the localisation of gold mineralisation
during, or immediately prior to, the first phase of granitoid intrusion.
Mineralisation at Golden Ridge is closely associated with an intrusion of biotite
granodiorite. Many of the prospects in this area correspond to irregular
magnetic anomalies which are inferred to mark zones of magnetite alteration
based on assessment of an adjoining anomaly to the west of Golden Ridge.
This anomaly, which is one of several broad magnetic features in the Mathinna
region, extends beyond the mapped contact aureole. The presence of biotite in
association with magnetite in this area indicates temperatures in excess of 400
degrees. These magnetic anomalies may indicate the presence of large alteration systems possibly related to granodioritic intrusives. The economic
potential of these areas should be assessed.
This study indicates that two distinct styles of gold mineralisation are present
iv
in northeast Tasmania. The mineralisation at Gladstone and in the main gold
corridor from Mangan a to Waterhouse is structurally controlled and apparently
unrelated to granitoids. At Lisle and Golden Ridge mineralisation is spatially
and genetically associated with granodioritic intrusives. No single set of criteria
for locating potentially mineralised sites across the entire region has been
established but analysis of detailed magnetic data can greatly assist in the
definition of zones of structural complexity or areas of alteration.